In injection molding apparatus of the type having a mold cavity and a core therein for molding of hollow plastic products of predetermined design, such as for example spouts for caulking cartridges, it is customary to utilize a gate valve assembly, positioned between mechanisms for supplying plastic molding compound under pressure from a source and the mold cavity, for monitoring and controlling the flow of plastic molding compound. These gate valve assemblies normally include a valve housing having passageways therein for receiving the plastic molding compound under pressure from the supply mechanisms and a gate of restricted dimensions at the bottom of the housing which communicates with the top of the mold cavity for injecting a jet of the plastic molding compound under a predetermined pressure into the mold cavity to impinge upon the upper end of the mold core and flow around the mold core therein to fill the cavity.
The gate of the gate valve assembly and the core within the mold housing cavity are usually aligned and the core is usually aligned in the center of the mold cavity so that desirably the jet of plastic molding compound injected by the gate of the valve assembly into the mold cavity will impinge upon the end of the core in such a manner as to result in an even distribution of the plastic molding compound around the core to flow symmetrically into the space around the entire length of the core to the bottom of the mold cavity.
However, in view of the high pressure of the jet of plastic molding compound injected from the gate of the valve assembly against the upper end of the core, the core will often be deflected out of alignment with the gate and within the mold cavity resulting in the flow of the molding compound on one side of the core to lead the flow of the molding compound on the other side of the core. This uneven flow of the plastic molding compound and deflection of the core produces uneven thicknesses in the wall of the hollow molded plastic product being formed and, more seriously, produces weld lines where two flow fronts of the plastic molding compound meet each other during filling of the mold cavity. These resulting weld lines produce weak areas in the resulting molded plastic product and thus product failure under many conditions of use.
In an effort to overcome these problems of core deflection resulting in uneven wall thicknesses and weld lines in the resulting molded plastic product, it has heretofore been proposed to utilize a gate valve assembly of the type in which the gate of restricted dimensions is widened by the use of a generally cylindrical gate and valve pin for opening and closing of the gate when flow of the plastic molding compound is desired for filling the mold cavity and when the flow of the plastic molding compound should be shut off after the mold cavity is filled. As a result of these widened or increased dimension gates, which are possible by the use of cylindrical gates and valve pins, a lower pressure jet of plastic molding compound is injected into the mold cavity to impinge upon the upper end of the mold core. However, even such reduction in pressure of the plastic molding compound being injected through the valve assembly gate does not always overcome the core deflection problem and thus uneven wall thicknesses and weld lines in the molded plastic product, particularly when the top of the core against which the plastic molding compound is being injected is relatively small or pointed, such as the case with a caulking cartridge spout, because uneven deflection forces are still produced by the impinging jet of plastic molding compound if any slight misalignment is present.
Moreover, the previously proposed gate valve assemblies utilizing a cylindrical gate and valve pin for reducing the pressure of the plastic molding compound jet have required a complicated control system for opening and closing of the gate of the valve assembly by raising the gate valve pin to open the gate and lowering the gate valve pin to close the gate. Although solutions to this problem of having an expensive and complicated control system for the gate valve pin have been proposed, such as suggested in prior U.S. Pat. No. 4,289,468 which provides a "self-regulating" gate valve using a specially designed gate vale pin, such specially designed gate valve pin enhances core deflection by injecting a jet of plastic molding compound which produces uneven forces on each side of the top of the core in the mold cavity.